The association of Norwalk virus and related 27nm viruses with acute, epidemic gastroenteritis is well established, but the full extent of the role of these viruses in enteric illness has been difficult to assess. Major advances in our understanding of the epidemiology of Norwalk virus were made this year using recombinant capsid protein from cloned Norwalk virus (provided to our laboratory by Dr. Estes, Baylor College of Medicine) as an antigen in an ELISA to detect serologic responses. With the availability of a relatively unlimited source of antigen, seroepidemiologic studies of Norwalk virus infection are now feasible. Preliminary serologic studies indicate that the Norwalk or a related virus is capable of infecting infants and young children more frequently than anticipated from prior limited studies. The association of such infections with illness is not known at present.